Network Security. Essentials. Chapter 2 презентация

Outline
Symmetric encryption
Block encryption algorithms
Stream ciphers
Cipher Block Modes

Symmetric Encryption
or conventional / private-key / single-key
sender and recipient share a common key
all classical encryption algorithms are private-key
was only type prior to invention of public-key in 1970’s
and by far most widely used

Crypto
Cryptology ⎯ The art and science of making and breaking “secret codes”
Cryptography ⎯ making “secret codes”
Cryptanalysis ⎯ breaking “secret codes”
Crypto ⎯ all of the above (and more)

Some Basic Terminology
plaintext - original message
ciphertext - coded message
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key
cryptology - field of both cryptography and cryptanalysis

Simple Substitution
Plaintext: fourscoreandsevenyearsago
Key:
Ciphertext:
IRXUVFRUHDAGVHYHABHDUVDIR
Shift by 3 is “Caesar’s cipher”
Plaintext
Ciphertext

Ceasar’s Cipher Decryption
Plaintext: spongebobsquarepants
Plaintext
Ciphertext
Suppose we know a Ceasar’s cipher is being used
Ciphertext: VSRQJHEREVTXDUHSDQWU

Not-so-Simple Substitution
Shift by n for some n ∈ {0,1,2,…,25}
Then key is n
Example: key = 7
Plaintext
Ciphertext

Cryptanalysis I: Try Them All
A simple substitution (shift by n) is used
But the key is unknown
Given ciphertext: CSYEVIXIVQMREXIH
How to find the key?
Only 26 possible keys ⎯ try them all!
Exhaustive key search
Solution: key = 4

Even-less-Simple Substitution
Key is some permutation of letters
Need not be a shift
For example
Plaintext
Ciphertext
Then 26! > 288 possible keys!

Cryptanalysis II: Be Clever
We know that a simple substitution is used
But not necessarily a shift by n
Can we find the key given ciphertext:
PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWAXBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPBFXFQVXGTVJVWLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAGFOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQHFOQPWTBDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJTQOTOGHFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVAFXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTOFFA

Cryptanalysis II
Can’t try all 288 simple substitution keys
Can we be more clever?
English letter frequency counts…

Cryptanalysis II
Ciphertext:
PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWAXBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPBFXFQVXGTVJVWLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAGFOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQHFOQPWTBDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJTQOTOGHFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVAFXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTOFFA
Ciphertext frequency counts:
Decrypt this message using info below

Comparison

Symmetric Cipher Model

Requirements
two requirements for secure use of symmetric encryption:
a strong encryption algorithm
a secret key known only to sender / receiver
mathematically have:
Y = E(K, X)
X = D(K, Y)
assume encryption algorithm is known
implies a secure channel to distribute key

Cryptography
can characterize cryptographic system by:
type of encryption operations used
substitution
transposition
product
number of keys used
single-key or private
two-key or public
way in which plaintext is processed
block
stream

Cryptanalysis
objective to recover key not just message
general approaches:
cryptanalytic attack
brute-force attack
if either succeed all key use compromised

Cryptanalytic Attacks
ciphertext only
only know algorithm & ciphertext, is statistical, know or can identify plaintext
known plaintext
know/suspect plaintext & ciphertext
chosen plaintext
select plaintext and obtain ciphertext
chosen ciphertext
select ciphertext and obtain plaintext
chosen text
select plaintext or ciphertext to en/decrypt

An encryption scheme: computationally secure if
The cost of breaking the cipher exceeds the value of information
The time required to break the cipher exceeds the lifetime of information

Brute Force Search
always possible to simply try every key
most basic attack, proportional to key size
assume either know / recognise plaintext

Symmetric Block Cipher Algorithms
DES (Data Encryption Standard)
3DES (Triple DES)
AES (Advanced Encryption Standard)

Data Encryption Standard (DES)
most widely used block cipher in world
adopted in 1977 by NBS (now NIST)
as FIPS PUB 46
encrypts 64-bit data using 56-bit key
has widespread use
has considerable controversy over its security

DES History
IBM developed Lucifer cipher
by team led by Feistel in late 60’s
used 64-bit data blocks with 128-bit key
then redeveloped as a commercial cipher with input from NSA and others
in 1973 NBS issued request for proposals for a national cipher standard
IBM submitted their revised Lucifer which was eventually accepted as the DES

DES Design Controversy
although DES standard is public, considerable controversy over design
in choice of 56-bit key (vs Lucifer 128-bit)
and because design criteria were classified
subsequent events and public analysis show in fact design was appropriate
use of DES has flourished
especially in financial applications
still standardised for legacy application use

Time to Break a DES Code (assuming 106 decryptions/μs)

Multiple Encryption & DES
clear a replacement for DES was needed
theoretical attacks that can break it
demonstrated exhaustive key search attacks
AES is a new cipher alternative
prior to this alternative was to use multiple encryption with DES implementations
Triple-DES is the chosen form

Triple DES

Triple-DES with Two-Keys
hence must use 3 encryptions
would seem to need 3 distinct keys
but can use 2 keys with E-D-E sequence
C = EK1(DK2(EK1(P)))
nb encrypt & decrypt equivalent in security
if K1=K2 then can work with single DES
standardized in ANSI X9.17 & ISO8732
no current known practical attacks
several proposed impractical attacks might become basis of future attacks

Why is the middle portion of 3DES a decryption rather than an encryption?
it is compatible with the older single DES by repeating the key.
It is more secure
Decryption is faster than encryption
no cryptographic significance
A
B
C
D
Submit

Triple-DES with Three-Keys
although no practical attacks on two-key Triple-DES have some concerns
Two-key: key length = 56*2 = 112 bits
Three-key: key length = 56*3 = 168 bits
can use Triple-DES with Three-Keys to avoid even these
C = EK3(DK2(EK1(P)))
has been adopted by some Internet applications, eg PGP, S/MIME

Origins
clearly a replacement for DES was needed
have theoretical attacks that can break it
have demonstrated exhaustive key search attacks
can use Triple-DES – but slow, has small blocks
US NIST issued call for ciphers in 1997
15 candidates accepted in Jun 98
5 were shortlisted in Aug-99
MARS
RC6
Rijndael
Serpent
Twofish
Rijndael was selected as the AES in Oct-2000
issued as FIPS PUB 197 standard in Nov-2001

The AES Cipher - Rijndael
designed by Rijmen-Daemen in Belgium
has 128/192/256 bit keys, 128 bit data
an iterative rather than feistel cipher
processes data as block of 4 columns of 4 bytes
operates on entire data block in every round
designed to be:
resistant against known attacks
speed and code compactness on many CPUs
design simplicity

AES Encryption Process

Comparison

Random Numbers
many uses of random numbers in cryptography
nonces in authentication protocols to prevent replay
session keys
public key generation
keystream for a one-time pad
in all cases its critical that these values be
statistically random, uniform distribution, independent
unpredictability of future values from previous values
true random numbers provide this
care needed with generated random numbers

Pseudorandom Number Generators (PRNGs)
often use deterministic algorithmic techniques to create “random numbers”
although are not truly random
can pass many tests of “randomness”
known as “pseudorandom numbers”
created by “Pseudorandom Number Generators (PRNGs)”

Random & Pseudorandom Number Generators

PRNG Algorithm Design
Purpose-built algorithms
E.g. RC4
Algorithms based on existing cryptographic algorithms
Symmetric block ciphers
Asymmetric ciphers
Hash functions and message authentication codes

Outline
Symmetric encryption
Block encryption algorithms
Stream ciphers
Cipher Block Modes

Stream Cipher Structure

Stream Cipher Properties
some design considerations are:
long period with no repetitions
statistically random
depends on large enough key, e.g. 128 bits
large linear complexity
properly designed, can be as secure as a block cipher with same size key
but usually simpler & faster

Linear feedback shift register
A 4-bit Fibonacci LFSR with its state diagram. The XOR gate provides feedback to the register that shifts bits from left to right. The maximal sequence consists of every possible state except the "0000" state.

RC4
a proprietary cipher owned by RSA DSI
another Ron Rivest design, simple but effective
variable key size, byte-oriented stream cipher
widely used (web SSL/TLS, wireless WEP/WPA)
key forms random permutation of all 8-bit values
uses that permutation to scramble input info processed a byte at a time

RC4 Security
claimed secure against known attacks
have some analyses, none practical
result is very non-linear
since RC4 is a stream cipher, must never reuse a key
have a concern with WEP, but due to key handling rather than RC4 itself

Outline
Symmetric encryption
Block encryption algorithms
Stream ciphers
Cipher Block Modes

The Most Important Modes
Electronic Codebook Mode (ECB)
Cipher Block Chaining Mode (CBC)
Cipher Feedback Mode (CFB)
Counter Mode (CTR)

Electronic Codebook Book (ECB)
message is broken into independent blocks which are encrypted
each block is a value which is substituted, like a codebook, hence name
each block is encoded independently of the other blocks
Ci = EK(Pi)
uses: secure transmission of single values

2021/3/4
Zimmerman Telegram
februar 13605
fest 13732
finanzielle 13850
folgender 13918
frieden 17142
friedenschluss 17149

2021/3/4
Zimmerman
Decryption
UK decrypt part of the telegraphy

Advantages and Limitations of ECB
message repetitions may show in ciphertext
if aligned with message block
particularly with data such as graphics
or with messages that change very little, which become a code-book analysis problem
weakness is due to the encrypted message blocks being independent
main use is sending a few blocks of data

Cipher Block Chaining (CBC)
message is broken into blocks
linked together in encryption operation
each previous cipher blocks is chained with current plaintext block, hence name
use Initial Vector (IV) to start process
Ci = EK(Pi XOR Ci-1)
C0 = IV
uses: bulk data encryption, authentication

Cipher Block Chaining (CBC)

Cipher FeedBack (CFB)
message is treated as a stream of bits
added to the output of the block cipher
result is feed back for next stage (hence name)
standard allows any number of bit (1,8, 64 or 128 etc) to be fed back
denoted CFB-1, CFB-8, CFB-64, CFB-128 etc
most efficient to use all bits in block (64 or 128)
Ci = Pi XOR EK(Ci-1)
C0 = IV
uses: stream data encryption, authentication

s-bit
Cipher FeedBack (CFB-s)

Advantages and Limitations of CFB
appropriate when data arrives in bits/bytes
most common stream mode
Limitation: need to stall while doing block encryption after every n-bits
note that the block cipher is used in encryption mode at both ends
errors propagate for several blocks after the error

Counter (CTR)
a “new” mode, though proposed early on
similar to OFB but encrypts counter value rather than any feedback value
must have a different key & counter value for every plaintext block (never reused)
Oi = EK(i)
Ci = Pi XOR Oi
uses: high-speed network encryptions

Counter (CTR)

Advantages and Limitations of CTR
efficiency
can do parallel encryptions in h/w or s/w
can preprocess in advance of need
good for bursty high speed links
random access to encrypted data blocks
provable security (good as other modes)
but must ensure never reuse key/counter values, otherwise could break (cf OFB)

Output Feedback Mode (OFB)